The overall goal of this project is to characterize the neuroendocrine mechanisms that underlie daily torpor, an extreme form of temperature regulation that evolved to help animals contend with limited food availability. The relation of torpor to overall energy balance and reproduction will be explored. Annual rhythms of daily torpor are well documented in the Siberian hamster (Phodopus sungorus), the model species in all experiments. Specific aims include: (1) determining the role of caudal brainstem and hypothalamic mechanisms in initiation of daily torpor; (2) specifying possible central neural systems activated during initiation of torpor with c-fos; (3) identifying the role of endogenous glucose during the initiation of torpor behavior; (4) evaluating the role of peripheral neural mechanisms in the initiation of torpor; (5) characterizing the neural mechanisms underlying testosterone inhibition of torpor; and (6) examining the counteractive nature of lactation and hyperprolactinemia on torpor.